Place potatoes in a large pot and cover with water. Bring to a boil and cook until fork tender, about 15 minutes. Drain, reserving about 1 cup of the potato water. Return potatoes to pot. Mash potatoes with a potato masher, add olive oil, about ½ of the reserved water, and season with salt and pepper. Stir to combine, adding more reserved water (or dairy-free milk) if needed to thin out the mashed potatoes. Once cooked, set aside.

Meanwhile, heat about a tablespoon of olive oil in a large skillet over medium heat. Add ground beef and cook until browned. Stir in onion and garlic and cook until fragrant. Add frozen mixed veggies and let cook until just tender.

Coat beef/veggie mixture with flour. Stir in tomato paste, broth, worcestershire sauce, italian seasoning, salt, and pepper. Bring to a simmer, then cook cook over low heat until the sauce reduces and thickens to your liking.

Preheat the oven to 350F. Transfer mixture to an oil-sprayed 9x13-inch (or similar size) casserole dish and top with mashed potatoes. If desired, use a fork to create texture or ruffles in the topping.

Bake for 30 minutes, or until golden on the top and bubbling around the edges. Once cooked, remove from the oven, let cool for about 5 minutes, then serve warm.